This invention relates to vertical form, fill and seal machines, and in particular to a vertical form, fill and seal machine and method of operating the machine in which variations in the velocity of the film fed through the machine do not unintentionally vary the lengths of the packages being made by the machine.
For many years, manufacturers of vertical form, fill and seal machines have constructed the machinery with a series of clutches and brakes for operating the interlinked portions of the machine. One such machine is described in U.S. Pat. No. 4,288,965 assigned to Hayssen Manufacturing Company, the disclosure of which is incorporated herein by reference. The '965 patent describes a vertical form, fill and seal machine which uses a measuring roll to meter the delivery of film to the machine to produce constant length packages. The particular system described in the patent uses a clutch and brake drive system in conjunction with a shaft encoder to operate the measuring rolls within a fixed portion of a 360.degree. bag making cycle.
The system of the '965 patent operates well when the machine is operated at a single delivery velocity for the film. However, when the speed of the film is changed, the number of degrees required to produce a package of a desired length must be changed, as well, in order to achieve that length. This is necessary because even though the measuring rolls are commanded to operate for the duration of a commanded pull signal, once the signal has ceased, the inertia of the measuring rolls causes an over-running situation. As the velocity of the rolls increases, the over-running situation worsens. While a modification of the commanded pull signal resolves the variation in package length, it is impossible to vary the velocity over a wide range without readjusting the commanded parameters for the machine. As a result, the machine, when operating with a clutch and brake drive system, normally is not utilized in situations where varying velocities occur.
However, with the advent of motion control systems, it is now possible to utilize mathematical relationships within the motion controllers used in such systems to automatically compensate for length variations that would otherwise occur due to inertia of the measuring rolls. Thus, it is now possible to accurately compensate for deceleration of the measuring rolls, which occurs outside the commanded pull signal, in order to produce packages of consistent length, without utilizing a shaft encoder or the like on the measuring rolls in order to determine the position of the rolls, and therefore the amount of film pulled.
In a typical vertical form, fill and seal machine of the nature of the invention, the portion of the 360.degree. bag making cycle in which the measuring rolls are operated has three phases, an acceleration phase where the speed of the film is accelerated from rest to a desired maximum or running velocity, a running velocity phase where the rolls are operated at a constant velocity, and a deceleration phase, where, after the commanded pull signal has ceased, the measuring rolls are decelerated to rest. In a typical machine, the example of which is utilized in connection with the present invention, the acceleration phase occupies one third of the time of the commanded pull signal, and the running velocity phase occupies the remaining portion, or two thirds, of the commanded pull signal. It is only the deceleration phase that will vary. If there is a constant deceleration, then the amount of over-pull will vary depending on the velocity from which the measuring rolls are halted. On the other hand, the deceleration can be variable, and, in that instance, the deceleration distance always remains the same. Therefore, the time of the deceleration will vary inversely to the value of the velocity from which the measuring rolls are halted.